I bring my phone on the trail to serve as a backup camera, a GPS, a notebook, the novel I'm reading, and so on. It's always in airplane mode while I'm Out There, because having marginal or no service drains the battery - so I carry it to be everything BUT a phone!
I now have a ruggedized phone that is just in a silicone sleeve. Before that I used one or another sort of IP68 case - Lifeproof, Otter Box, or Seidio.
Immersion-test the case before you use it.